The invention concerns a hydraulic steering arrangement with a directional valve and a metering pump unit having two hydraulically parallel connected and mechanically parallel operable metering pumps, each having a toothed ring with inner toothing and a gear wheel with outer toothing being able to perform rotary and orbiting movements in said toothed ring.
A hydraulic steering arrangement of this kind is for example known from DE 195 11 501 A1 or DE 196 07 0674 A1.
Such steering arrangements are particularly applied in connection with the steering of vehicles. In this connection, the metering pump unit is connected with a steering motor via a directional valve. The directional valve is connected with a steering handwheel or a similar device. When the steering handwheel is turned, the directional valve is displaced in the desired direction. The metering pump unit supplies hydraulic fluid until the steering motor has reached the desired position. In this state the directional valve is displaced again, so that no further supply of hydraulic fluid takes place.
During normal, that is, undisturbed, operation, both metering pumps are active. Accordingly, they can supply a relatively large volume of hydraulic fluid, which enables a quick reaction of the steering motor to movements of the steering handwheel.
When the pressure on the pump connection, which usually provides the pressurised hydraulic fluid, should for some reason decrease or break down, for example because the responsible supply pump is defective or its drive has failed, the metering pumps can also be used as auxiliary pumps in a so-called xe2x80x9cemergency operationxe2x80x9d. In this case, the metering pumps are thus used to pressurise the hydraulic fluid. The required energy for this purpose must be provided via the steering handwheel, that is, usually by means of human muscle power.
In order to facilitate the work for the operator in this connection, the above patent documents disclose the use of only one of the two metering pumps for the fluid transport, disconnecting or short-circuiting the other pump. The operator then only has to operate one metering pump. For the same deflection of the steering motor, he has to turn the steering handwheel a longer distance. The amount of power required, however, is smaller.
In the known case, one of the two metering pumps is driven via a cardan shaft, which is in rotary connection with the gear wheel via a spline connection. Normally, this cardan shaft is required, as the metering pump is arranged eccentrically to other parts of the steering arrangement, which can also rotate. The tapered structure or the inner toothing of the gear wheel also serves the adoption of an additional shaft, by means of which a torque from the gear wheel of the first metering pump is transferred to the gear wheel of the second metering pump. Also this second gear wheel has a corresponding tapered structure on its inside.
It has now been observed that metering pumps of this kind do not always have as long a life as desired.
Accordingly, the task of the invention is to prolong the life of the metering pumps.
In a steering device as mentioned in the introduction, this task is solved in that a through-going driving part for both gear wheels is provided.
Accordingly, the gear wheels of both metering pumps are always driven by the same driving part. Therefore, their movements are to a high degree synchronised. This is a clear improvement in relation to the known cases. Here, the at least double tapered meshing between the first and the second gear wheel always made a small tolerance unavoidable. In unfavourable cases, this tolerance could cause commutation displacements to occur, which again would lead to pressure peaks. The pressure peaks again mean a risk to the gear sets, and in particular for the gear set which is driven by the second shaft, which is normally also made as a cardan shaft. When, however, only one single driving part is used, common for both gear wheels, these problems do not occur. It is substantially easier to keep the commutation in accordance, so that pressure peaks can be avoided or at least substantially reduced.
Preferably, the engagement ratios between the driving part and the gear wheel of each metering pump are equal. In a relatively simple embodiment, this can be realised in that the engagement geometry of both gear wheels is kept the same. Thus, the driving part is provided with uniform engagement geometry over its whole axial length. Also the two gear wheels have an identical engagement geometry. Thus, additionally to the use of a common driving part, the same engagement geometry or the same engagement behaviour, respectively, ensures that both gear wheels are driven in practically identical ways.
Preferably, the driving part is made of part of the directional valve. In this case, a tolerance will not occur in a possible connection between the driving part and the corresponding part of the directional valve. On the contrary, the directional valve continues into the metering pump with the driving part.
Preferably, the directional valve has an inner rotary slide and an outer rotary slide, which are rotatable in relation to each other, the outer rotary slide extending eccentrically through the gear wheels of the metering pump, and the gear wheels being in torque-transmitting connection with the outer rotary slide. At the same time, the gear wheels rotate and orbit in the corresponding toothed rings. Thus, with their centre they describe a rotation around a rotation axis of the rotary slides. It can now be ensured that between the outer rotary slide and the gear wheels a connection exists, which transmits the rotary torque, not, however, the orbiting movement, to the rotary slide. In this case, the rotary slide can be arranged concentrically around the toothed ring of the metering pump.
Preferably, each gear wheel has an inner toothing, which engages with an outer toothing of the outer rotary slide, the engagement being limited to a circumferential area, which travels in the circumferential direction when the gear wheels are orbiting. With this relatively simple measure, the movement transmission from the gear wheel to the rotary slide can be limited to a pure rotary movement. The movement is transmitted via the engagement between the inner toothing and the other toothing. The orbiting movement of the gear wheel is permitted, as the rotary slide always only engages with the gear wheel on part of its circumference. The remaining part is free and can thus be used for the orbiting movement.
Preferably, the outer rotary slide has a reduced diameter on the section arranged close to the gear wheels. Thus, the outer dimensions of the steering arrangement can be kept exactly as small as in the known cases mentioned in the introduction. In fact, the same housing can also be used. Merely the metering pump section has to be changed a little.
Preferably, the inner rotary slide bears axially with one end on a wall of the outer rotary slide projecting radially inwards. Thus, the inner rotary slide is fixed in the outer rotary slide, so that the overlapping of the control openings and recesses can be maintained over a longer period with an extremely high accuracy.
It is also advantageous when the driving part passes through one of the gear wheels in its full length and projects at least halfway into the other gear wheel. Thus, it is obtained that the gear wheels do not tilt in relation to the rotary slide, that is, do not incline out of their rotation level. Also this measure serves the purpose of keeping the wear at a low level.